FIG. 2. Oral self-administration of caffeine capsules
in each of three male volunteers with histories of heavy coffee drinking
who resided in a residential laboratory. Subjects had 13 opportunities each
day to self-administer either a caffeine (100 mg) or a placebo capsule.
Vertical lines indicate a change in condition: Either novel double-blind
color codes associated with caffeine and placebo capsules were introduced
or a period of capsule unavailability was initiated. (Redrawn from ref.
50.)